I will point to the fact that currently drupal will not support mysql's replication technology since we haven't implemented a concept of slave/master...
Two quick ways I can think of adding it are... 1) further db abstraction ala, db_select, db_insert, db_update..... 2) and using a if (!strpos('SELECT', $query)) { db_set_active('master') } in db_query.
with 1) you may get slightly better performance since you aren't constantly parsing strings, but you start making some major changes to drupal's db_abstraction layer.
Are you suggesting that we add a connection pooling mechanism to Drupal? I'm not convinced that is a good idea. There are both software and hardware load balancer that do exactly that. You open a MySQL connection to db-server.example.com (the load balancer) which map it onto db-server-1.example.com, db-server-2.example.com, etc. These load balancers can do 'health checks' to see if the database servers are still running, whether they are in a consistent state, whether they are properly replicated, etc. Good health checks can be complicated and therefore this could be tricky to implement in PHP ... -- Dries Buytaert :: http://www.buytaert.net/